Despite recent successes, a major impediment to progress in clinical transplantation continues to be allograft rejection and morbidity and mortality resulting from non-specific immunosuppression. Therefore, achievement of long lasting donor-specific unresponsiveness to foreign antigens would constitute an important clinical advance. This FIRST proposal will examine the potential use of gene transfer technology to induce antigen-specific transplantation tolerance. More precisely, these studies will determine whether tolerance can be induced in lethally irradiated mice reconstituted with autologous bone marrow cells into which xenogeneic MHC genes have been introduced and expressed. Retroviral expression vectors, which contain the human HLA-A2 gene will be constructed and introduced into packaging cell lines. Colonies which produce high titer, replication-defective recombinant retroviruses will then be co-cultured with mouse bone marrow cells to attain optimal gene transfer. Irradiated C57B1/6 (B6) mice will then be transplanted with autologous bone cells transformed to express the human HLA-A2 gene. Long-term bone marrow reconstituted B6 mice will then be examined for immunologic reactivity to the HLA-A2 antigen by 1) in vitro immunologic assays using lymphocytes for stimulation from cell lines or transgenic mice expressing the HLA-A2 gene on the B6 background and 2) by transplantation of skin grafts from HLA-A2 transgenic or third-party control donor mice onto reconstituted B6 recipients. The successful completion of these experiments should provide insight into xenogeneic tolerance induction and will provide an impetus to further work designed to study tolerance across more complicated histocompatibility barriers for clinical application.